"I am very excited to share with you guys that William and I got engaged over the weekend," Conrad wrote in a blog post on Sunday alongside a photo of her diamond ring.
"I am beyond thrilled! Get ready for lots more wedding content [on my blog] as we begin the planning process."
The bride-to-be later posted an "update" of her upcoming nuptials on her website.
"William and I are overwhelmed by all of your warm wishes. Thank you so much for sharing in this special moment :-)," Conrad added.
The reality TV star apparently has the support of all her friends and family.
"Everyone is so happy for her. He's such a great guy. They are perfect for each other!" a source toldPeople.
Conrad began dating Tell, a law student, last year.
Conrad left The Hills -- which MTV ended after six seasons in 2010 -- during its fifth season in 2009, ending a five-year period in which she starred on both The Hills and its Laguna Beach predecessor, which followed her as a 17-year-old high school senior in Laguna Beach, CA.